The present invention is directed generally to radio communication systems and, more particularly, to techniques and structures for presetting transmit power levels in radio communication systems.
Traditionally, radio communication systems have employed either Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) to allocate access to available radio spectrum. Both methods attempt to ensure that no two potentially interfering signals occupy the same frequency at the same time. For example, FDMA assigns different signals to different frequencies. TDMA assigns different signals to different timeslots on the same frequencies. TDMA methods reduce adjacent channel interference through the use of synchronization circuitry which gates the reception of information to prescribed time intervals.
In contrast, Code Division Multiple Access (CDMA) systems allow interfering signals to share the same frequency at the same time. More specifically, CDMA systems "spread" signals across a common communication channel by multiplying each signal with a unique spreading code sequence. The signals are then scrambled and transmitted on the common channel in overlapping fashion as a composite signal. Each mobile receiver correlates the composite signal with a respective unique despreading code sequence, and thereby extracts the signal addressed to it.
The signals which are not addressed to a mobile receiver in CDMA assume the role of interference. To achieve reliable reception of a signal, the ratio of the signal to the interference should be above a prescribed threshold for each mobile station (referred to as a "required signal-to-interference" level, or SIR.sub.req). For example, as shown in FIG. 1A, consider the case where three mobile stations receive, respectively, three signals from the common CDMA communication band. Each of the signals has a corresponding energy associated therewith--namely energy levels E1, E2 and E3, respectively. Also, present on the communication band is a certain level of noise (N). For the first mobile station to receive its intended signal, the ratio between E1 and the aggregate levels of E2, E3 and N must be above the first mobile's required signal-to-interference ratio.
To improve the signal to interference ratio for a mobile, the energy of the signal is increased to appropriate levels. However, increasing the energy associated with one mobile station increases the interference associated with other nearby mobile stations. As such, the radio communication system must strike a balance between the requirements of all mobile stations sharing the same common channel. A steady state condition is reached when the SIR requirements for all mobile stations within a given radio communication system are satisfied. Generally speaking, the balanced steady state may be achieved by transmitting to each mobile station using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high levels raises interference experienced at each mobile receiver, and limits the number of signals which may be successfully communicated on the common channel (e.g. reduces system capacity).
A steady state condition must be adjusted for various changes within the mobile communication system. For instance, when a new mobile station enters a communication cell, it will create additional interference within the system. For example, as illustrated in FIG. 1B, the introduction of a fourth mobile station to the steady state condition depicted in FIG. 1A imposes a new signal on the common communication channel with energy E4. This new signal energy E4 adds to the aggregate interference experienced by the first through third mobile stations already in the cell. Accordingly, in order to satisfy the required signal-to-interference ratios of the first through third stations, the power associated with the first three mobile stations E1-E3 may have to be adjusted accordingly. The same disrUptive effect may be experienced when a mobile station which was previously located within the boundaries of the radio communication cell switches from a passive state to an active state to transmit or receive a message on the common channel.
The steady state condition is also disrupted when a mobile station leaves the radio communication cell. For example, if the steady state condition shown in FIG. 1A is disrupted by the third mobile station leaving the radio communication cell, the signal-to-interference ratio of the remaining two mobile stations will be improved by the absence of the energy E3 on the common channel, as shown in FIG. 1C. Accordingly, the power of signals E1-E2 can be decreased to ensure efficient use of the common communication channel. Again, this same effect may be achieved when the third mobile station within the radio communication cell switches from active to passive state (e.g. by terminating its call).
Still another disruption of the steady state may occur when one or more mobile stations within a radio communication cell changes its operating characteristics. For example, as illustrated in FIG. 1D if the third mobile station switches from a low data-rate mode of communication to a high data-rate mode of communication, the remaining two mobile stations within the cell will experience increased levels of interference. To counteract the increased levels of interference in the communication band, the system may have to adjust the power levels E1 and E2. The reverse effect may occur when a mobile station switches from a high data-rate mode to a low data-rate mode.
Prior CDMA-based systems use one or more power control loops to appropriately adjust the power levels of signal transmission within the system to counteract the above described disruptions to the steady state condition. According to one exemplary prior technique, for the downlink the mobile station monitors the strength at which it receives signals from the base site. If the signals are too weak, the mobile station transmits a message to its associated base station informing the base station to increase the power at which it transmits to the mobile station. The base station will respond accordingly. However, over time, the base will "tease" the mobile station by slowly decreasing the power to the mobile station until the base station is informed by the mobile station to once again increase the power of transmission to the mobile station. This ensures that the base station is not communicating with the mobile stations using power levels which are unnecessarily high.
For example, in the case of FIG. 1B where a fourth mobile station enters a cell, the other mobile stations may instruct the base station to increase the level of power to the mobile stations. The base station will respond accordingly by increasing the power by one increment. If still insufficient to satisfy the mobile station's SIR requirements, the mobile stations will repeat their message to the base station, once again requesting the base station to increase the level at which it transmits messages to the mobile stations. This procedure may be repeated through a series of communications between the base and the mobile stations. If the base "overshoots" the power requirements of the mobile stations, it may have to decrease the power levels to the mobile stations.
The iterative nature of this adjustment procedure results in a delay between the time at which a disruption in the interference situation occurs and a time at which the steady state condition is restored. As such, this technique is not well suited for particularly large disruptions to a radio communication system, such as when a high data-rate user suddenly enters a cell comprising only a few mobile users. In this circumstance, as shown in FIG. 1E, the introduction of a new data user at time t=0 will cause a temporary drop in SIR level for user j, which in turn may lead to erroneous signal reception. Such transient peaks in SIR level are particularly common in systems with bursty high data rate users (which are characterized by their discontinuous on-and-off transmission).
It is therefore an exemplary objective of the present invention to adjust the power levels associated with a plurality of mobile stations, in response to the changing needs of the plurality of mobile stations, without resorting to the above described iterative procedure.